The present invention relates to methods and apparatus for oxidizing nitric oxide (NO) present in a gas stream and converting SO3, VOCs, and other pollutants using oxidizing compounds thus forming compounds that can be more readily removed from the gas stream by conventional means. In particular, the present invention employs microwave energy to activate oxidizing compounds that oxidize NO and that convert other gaseous components into more desirable compounds to allow removal of these gaseous components using pollution control devices such as wet scrubber systems.
Carbonaceous fuels such as coal, oil, and other low molecular weight hydrocarbons are widely used in a range of combustion operations including utility boilers, industrial furnaces, and the like. Combustion of these carbonaceous fuels at high temperatures is considered to be beneficial due to a reduction in carbon monoxide emission and more efficient combustion. However, high temperature combustion also promotes the production of nitrogen oxides, also referred to as NOx, in the form of nitric oxide, nitrogen dioxide and the like. Nitrogen oxides are known to form ground level ozone, to comprise a major irritant in smog and to be a significant contributor to acid rain.
Typically, nitrogen oxides can be present in effluent combustion gas mixtures at concentrations ranging from about 200 to about 1,000 parts per million (ppm). Nonetheless, these relatively small concentrations are significant due to the rapid rate at which effluent combustion gases are produced. For example, a typical 500 megawatt power plant emits approximately 1,000,000 cubic feet per minute of combustion gases into the atmosphere.
The detrimental impacts of effluent emissions continues to pose a significant environmental concern. For example, despite efforts to the contrary, ground level ozone concentrations still exceed federal guidelines in many urban areas in the United States. Under the Clean Air Act and its amendments, such ozone nonattainment areas must implement stringent NOx emissions regulations. These minimal emissions levels are attainable only by treatment of the flue gases exiting the combustion chamber.
The effluent streams of many large utility boilers are currently fitted with extensive scrubbing systems, designed for the removal of sulfur dioxides. It would be highly advantageous to use this existing scrubbing equipment to remove NOx from the effluent, as well. Unfortunately, scrubbing processes are generally only effective at scrubbing higher valence nitrogen oxides, such as nitrogen dioxide.
A variety of techniques have been proposed to oxidize nitric oxide and other oxidizable pollutants. For example, nitric oxide can be reacted with an oxidizing compound, such as hydrogen peroxide, particularly at elevated temperatures. In particular, it is known in the art that hydrogen peroxide can be injected directly into either the combustion chamber or the exiting flue gases.
However, direct injection of hydrogen peroxide is problematic. Direct injection of hydrogen peroxide into the combustion chamber can suppress the combustion temperature, resulting in a loss of combustion efficiency. Also, direct injection of hydrogen peroxide into the combustion chamber can result in the decomposition of a significant percentage of the hydrogen peroxide, thus reducing the quantity of hydrogen peroxide available to participate in the oxidation reaction. In contrast, injection of hydrogen peroxide into the flue gases can reduce the reactivity of the hydrogen peroxide, due to the lower temperature of the effluent gas traveling down the flue. In situ heating methods, such as spraying the hydrogen peroxide onto a heated surface placed within the flue, e.g. a hot plate, can aid in the vaporization and reactivity of hydrogen peroxide. However, such in situ contact heating methods also tend to decompose a large percentage of the injected hydrogen peroxide.
The present invention provides a method and apparatus that uses highly reactive forms of oxidation compounds to oxidize lower valence compounds such as nitric oxide, from gaseous streams, particularly combustion effluent streams, without sacrificing combustion temperature or decomposing significant amounts of the oxidant. Other compounds that may be converted in accordance with the invention include SO3 and volatile organic compounds. The present invention yields oxidized effluent components that can be readily removed by conventional pollution abatement processes such as wet scrubbing. This invention is based in part on the discovery that the reactivity of oxidizing compounds, such as hydrogen peroxide, can be increased by irradiation with microwave energy without inducing significant decomposition of the oxidizing compound or loss of combustion efficiency.
In one particularly advantageous aspect of the invention, a method for oxidizing nitric oxide is provided including the steps of introducing an oxidizing compound into an oxidation chamber, activating the oxidizing compound by exposing the oxidizing compound to microwave energy, and contacting the activated oxidizing compound with a gaseous stream containing nitric oxide to convert the nitric oxide into a scrubbable form of nitrogen oxide such as nitrogen dioxide. The oxidizing compound can be dissolved in an oxidizing solution. The oxidizing compound can be introduced into the oxidation chamber by a variety of processes including nebulizing, using a wicking system, or combinations thereof. The activated oxidizing compound can be in the form of a vapor, gas, or free radical. In one particularly advantageous embodiment, the oxidizing compound is hydrogen peroxide. In addition, the oxidation process can be optimized through use of a closed loop system that monitors the residual nitric oxide in the oxidized gas stream and controls the amount of oxidizing solution introduced into the oxidation chamber and/or the microwave energy input. The oxidized nitric oxide formed by the methods of the present invention can then be removed from the oxidized gas stream by processes such as wet scrubbing.
The invention further relates to an apparatus for performing the methods of the present invention. In one particularly advantageous embodiment, an apparatus for oxidizing a nitric oxide-containing gaseous stream is provided, including an oxidation chamber for contacting nitric oxide with an activated oxidizing compound, a corrosion shield that lines the oxidation chamber, at least one distributor to introduce the oxidizing compound into the oxidation chamber, and at least one microwave generator. The oxidation chamber can optionally be fitted with baffles for facilitating the mixing of the nitric oxide-containing gaseous stream and the activated oxidizing compound. In one advantageous embodiment, the oxidation chamber is comprised of a cylindrical member.
Yet another aspect of the invention relates to an effluent gas oxidation system for forming highly activated oxidizing compounds, such as hydrogen peroxide, in a compact system that will easily allow retrofitting of existing exhaust gas piping and stacks. In addition, aspects of the present invention are directed to an effluent gas emission system comprising a combustion process flue into which an effluent gas oxidation system has been secured upstream of a wet scrubber. A benefit of the present invention is that the existing effluent gas flues of combustion processes can be retrofitted with the present gas oxidation system, thus improving pollution abatement with minimal modifications to existing capital.
The present invention thus provides a reliable, cost effective means of oxidizing flue gas pollutants, such as nitric oxide, using activated forms of oxidizing compounds. These oxidized flue gases can then be effectively removed from the gas stream using existing pollution abatement devices such as wet scrubbers.
Further understanding of the processes and systems of the invention of the invention will be understood with reference to the brief description of the drawings and detailed description which follows herein.